Conventionally, to facilitate a stepwise transition from third-generation (3G) mobile communications to fourth-generation (4G) mobile communications, Long Term Evolution (LTE: supper 3G) has been developed as a high-speed mobile communications standard. While this LTE is positioned as an extension of International Mobile Telecommunications-2000 (IMT-2000), for the purpose of a smooth transition to 4G, wireless interfaces and Radio Access Controller (RAC) architectures are under drastic review from 3G systems. As a result, with compression of delay time and improvement in communication rate, LTE achieves a data communication rate of downlink 300 megabits per second at maximum and uplink 50 megabits per second at maximum.
Here, an example of the configuration of a mobile communication system using LET is explained with reference to FIG. 1. As depicted in FIG. 1, in the mobile communication system using this LTE, a plurality of wireless base stations are directly connected to a gateway apparatus (Evolved Packet Core: EPC) connected to a core network, and a Radio Network Controller (RNC) that controls these wireless base stations in a centralized manner in a conventional 3G system is omitted. Each wireless base station manages a plurality of communication areas (hereinafter, referred to as “cells”) to control wireless communication via any of the managed cells. In the mobile communication system using LTE, when a mobile station moves across cells (such movement is hereinafter referred to as “handover”), a connection with the move-origin cell is once cut off, and then a connection with the move-destination cell is established. The same goes for the case in which a mobile station moves across wireless base stations. For example, as depicted in FIG. 1, when a mobile station in wireless communication via a cell B managed by a wireless base station X moves to a cell C managed by a wireless base station Y, a connection with the cell B is cut off, and then a connection with the cell C is established.
When such a mobile station moves across wireless base stations, a connection between a cell managed by a wireless base station of move destination and the mobile station may fail. At this time, a wireless base station of move origin makes a reconnection under the initiative of a network system in cooperation with the wireless base station of move destination.
Also, a mobile communication system has been known in which, when a connection between the cell managed by the wireless base station of move destination and the mobile station fails, a base station performing handover control recognizes in advance the occurrence of a ping-pong phenomenon of repeating a reconnection until a connection between the cell managed by the wireless base station of move destination and the mobile station becomes successful, thereby preventing such a ping-pong phenomenon.
Here, an example is explained by using FIG. 2. in which a wireless base station performs handover control when the mobile station moves across wireless base stations in the mobile communication system using LTE. As depicted in FIG. 2, a handover request message including a plurality of handover-enable candidate cells at possible move destinations of the mobile station is transmitted to a wireless base station X currently in communication. Then, the wireless base station X selects one by one from the candidate cells included in the received message to cause a wireless base station Y managing the selected candidate cell to prepare handover allowing wireless communication via the candidate cell, and then receives the result indicative of either success or failure of the handover preparation. Specifically, the mobile station transmits to the wireless base station a message for requesting handover with a list containing a plurality of handover-enabled candidate cells (C, D, E, F) and, upon reception of this message, the wireless base station X selects a highest-priority candidate cell C and transmits to the wireless base station Y managing the selected candidate cell C a handover request for the candidate cell C. When a handover preparation for the candidate cell C fails, the wireless base station Y notifies the wireless base station X that the handover preparation for the candidate cell C fails. Upon reception of this failure notification for the handover request for the candidate cell C, the wireless base station X selects a next candidate cell D from among the candidate cells, and then transmits to the wireless base station Y managing the selected candidate cell D a handover request for the candidate cell D. When the wireless base station Y fails a handover preparation for the candidate cell D, the wireless base station X receives a notification indicative of a failure of the handover request for the candidate cell D, further selects a next candidate cell E, and transmits to a wireless base station Z managing the selected candidate cell E a handover request for the candidate cell E. When a handover preparation for the candidate cell E is successful, the wireless base station Z notifies the wireless base station X that the handover preparation for the candidate cell E is successful. The wireless base station X then notifies the mobile station making the handover request that the handover preparation for the candidate cell E managed by the wireless base station Z has been completed. The mobile station then performs wireless communication via the cell E managed by the wireless base station Z.
The conventional technology explained above is disclosed in, for example, Japanese Laid-open Patent Publication No. 2007-295318 and Japanese National Phase PCT Laid-open Publication No. 2007-527176.
However, in handover control of wireless base stations when the mobile station moves across the wireless base station in the mobile communication system using LTE, when a wireless base station fails a handover preparation, the number of messages exchanged among the wireless base stations is disadvantageously increased to put a load on the network. That is, the wireless base station receiving from the mobile station moving across the wireless base stations a handover request message including a plurality of handover-enabled candidate cells transmits a handover request to one of the received candidate cells, and then receives the result for every transmission. Therefore, the wireless base station communicates two messages, that is, transmission and reception, for each candidate cell until a success notification comes from a candidate cell. Therefore, the number of messages is increased when the number of failure notifications from the candidate cells is large, thereby putting a load on the network.
In particular, when many mobile stations simultaneously perform handover, for example, the number of messages exchanged among the wireless base stations is more significantly increased. That is, in the mobile communication system using LTE, as explained above, since the wireless base station controls wireless communication via a cell, the number of occasions of movement across wireless base stations is surely larger than the number of occasions of movement across RNCs in the current 3G system, thereby increasing the number of messages among the wireless base stations.